Cellulose ester film has been employed as a photographic negative film support, and in polarizing plates as a film which protects polarizers employed in liquid crystal displays, due to its high transparency, low birefringence, and ease of adhesion to polarizers.
In recent years, the production amount of liquid crystal displays has markedly increased due to the thin depth and light weight, and the demand is increasing. Further, television sets, which employ a liquid crystal display, exhibit features such as thinness and light weight. Thereby, large-sized television sets, production of which was not possible by employing Braun tubes, have been produced. Along with that trend, demand for polarizers and polarizer protecting films has been increasing.
Heretofore, these cellulose ester films have been produced mainly employing a solution-casting method. The solution-casting method, as descried herein, refers to a film forming method in which a solution prepared by dissolving cellulose ester in solvents is cast to form film and solvents are evaporated and dried to produce film. The film which is cast employing the solution-casting method exhibits high flatness, whereby by employing the resulting film, it is possible to produce uniform and high image quality liquid crystal displays.
However, an inherent problem of the solution-casting method is the necessity of a large volume of organic solvents followed by a high environment load. The cellulose ester film is cast employing halogen based solvents which result in a high environment load, due to its solubility characteristics. Consequently, it has particularly demanded to reduce the amount of used solvents, whereby it has been difficult to increase the production of cellulose ester film employing the solution-casting method.
Accordingly, in recent years, experiments have been conducted in which cellulose ester is subjected to melt-casting for the use of silver salt photography (Patent Document 1) and as a polarizer protective film (Patent Document 2) without using an organic solvent. However, cellulose ester is a polymer which exhibits a very high viscosity when melted and also exhibits a very high glass transition point. In order to lower the melt viscosity and glass transition point of organic polymers such as cellulose ester, it was found this that addition of plasticizers is effective.
In the above Patent Documents 1 and 2, employed are phosphoric acid ester plasticizers such as triphenyl phosphate or phenylenebisdiphenyl phosphate. In Patent Documents 3 and 4, saccharide derivatives are disclosed as plasticizers other than phosphoric acid ester plasticizers and used in the cellulose ester. However, even by using the known plasticizers, the viscosity decreasing effect thereof are insufficient for melt casting the cellulose ester. As a result, the casting speed of the cellulose ester from the dies will be small to result in decreased productivity compared with a solvent casting method. The cellulose ester extruded from the dies and cast on a cooling drum or a cooling belt is had to be leveled. It tends to be solidified in a short time after being extruded. Therefore, it was revealed that this method has a problem that the flatness of the obtained film is lower than the film produced by a solvent casting method.
On the other hand, it is an efficient method to increase the melting temperature for decreasing the melt viscosity. However, applying a high melting temperature will impose a disadvantage of decomposition or evaporation of the cellulose ester, the plasticizer, and other additives during melt casting. The prevention of decomposition or evaporation was still insufficient by the conventional known methods.
During the preparation of a polarizing plate by adhering a polarizer protective film and a polarizer, the cellulose ester film is dipped in an alkali solution of a high density and a temperature in order to easily coating the adhesive agent to the cellulose ester film. The surface of the film is saponified by this process and it becomes more hydrophilic. After this process, the adhesive agent is applied to the surface of the film and the polarizer is adhered to it. It was found that since the conventionally known methods has a problem of the robustness of the cellulose ester film after saponified, it will cause a hindrance for a continuous production of the polarizing plate or the adhesion between the cellulose ester film and the polarizer will be deteriorated.    Patent Document 1: JP-A No. 06-501040    Patent Document 2: JP-A No. 2000-352620    Patent Document 3: JP-A No. 2005-515285    Patent Document 4: JP-A No. 2006-265301    Patent Document 5: JP-A No. 2007-138121